A belt type continuously variable transmission in which the rotation speed of an input shaft connected to a drive source is varied continuously and transmitted to an output shaft is known. A belt type continuously variable transmission is constituted by a primary pulley, a secondary pulley, and a belt wrapped around the pulleys, and by varying the width of each pulley, the contact radius between the belt and the pulley is varied, leading to variation in a speed ratio. Oil pressure required to vary the pulley width is supplied by an oil pump and regulated as a line pressure by a solenoid or the like. This line pressure is further regulated by a spool or the like and then supplied.
By setting the line pressure low, an improvement in fuel economy is achieved, but when the line pressure decreases below an oil pressure (primary pressure) required for the primary pulley and an oil pressure (secondary pressure) required for the secondary pulley, the speed ratio varies.
JP2001-99280A published by the Japan Patent Office in 2001 discloses a technique of reducing the line pressure while keeping it higher than the primary pressure and secondary pressure by feedback-controlling the actual line pressure to a target line pressure.